This invention relates to the protection of electronic circuits from undesired effects due to static electrical discharges and in particular to the protection of the electronic circuits of postal franking meters.
Static electrical charge is often generated in modern office environments due to the low humidity levels maintained by air conditioning and frictional engagement between carpets and the shoes of personnel working in the office. Consequently, when personnel use equipment incorporating electronic circuits, static electricity is often discharged to the equipment. In some electronic equipment such discharges may result merely in momentary mal-function of the circuits with no long term ill effects. However in other electronic equipment, such as equipment carrying out data processing functions, a momentary mal-function may result in corruption of data being stored or processed with the result that subsequent operation of the equipment would be affected by such corruption of the data. Corruption of data is of particular concern in connection with equipment such as postal franking meters in which electronic circuits carry out accounting functions related to usage of the meter for franking mail items and are required to maintain reliably accurate accounting records. The records typically include the accumulated value used in franking mail items and the value of credit remaining available for use in further franking operations. Since these accounting records provide the basis on which the user is charged for usage of the meter and on which the postal authority receives revenue for postage it is very important to ensure that these possible discharges of static electricity do not result in mal-functioning of the electronic circuits of the postal franking meter.
While protection from static electricity discharges can be provided in some equipment by housing the equipment in a casing which is electrically conductive or has an electrically conductive layer entirely surrounding the circuitry, difficulties arise in using this form of protection with equipment which is required to provide interfaces such as a digital display and a keyboard between the equipment and a user. The display requires the provision of a transparent window in order that it can be viewed by the user and the keyboard requires the provision of an aperture or apertures in the casing through which the keys project.